If you have a Win8 install ISO and you make a bootable FAT32 USB drive containing the files from the ISO, you can boot it in both MBR\CSM and UEFI mode.

The drive must be formatted as FAT32 in order to UEFI boot.

However, if the \sources\install.wim is over 4GB (as in the case of an AIO ISO), it cannot be placed on a FAT32 partition as the maximum file size for FAT32 is 4GB.

You can split the install.wim into 3GB-sized install.swm and install2.swm files using DISM or a Wim Splitting tool (as in WinToolKit). However, if you do this and boot from it (in MBr or UEFI mode) you will get a 'Windows cannot find the Microsoft Software License Terms. Make sure the installation sources are valid and restart the installation.' error just after entering in the 5x5 Product Key.

The solution is to add a \sources\ei.cfg file which suppresses the Product Key prompt and which also allows you to choose which version you want from the install.wim.

Instructions

1. Make a single, Primary partition FAT32 USB drive in the normal way (e.g. RMPrepUSB, Rufus, Windows 7/8, etc.) with a bootmgr boot sector, etc. (or you can add grub4dos or whatever to make it MBR boot)

2. Extract the files from your Windows 8 source ISO to an empty folder on your system hard disk

3. Split the \sources\install.wim file into \sources\install.swm and \sources\install2.swm, etc, (WinToolKit - Wim Splitter is really easy to use for this!)

Usually Core (index 1) and Pro (index 2) are in the same install.wim file (at least for unmodified ISO) in all retail version, valid for x86 and x64 DVD's or Isos. So yes depending of the serial you use you can install Core or Pro (there are some generic serials just to install, but not good for validate), unless there is an EI.cfg file in sources directory like this for Core:

[EditionID]
Core
[Channel]
Retail
[VL]
0

Or like this for Pro:

[EditionID]
Professional
[Channel]
Retail
[VL]
0

Or like this for let you select:

[EditionID]
[Channel]
Retail
[VL]
0

Using the mentioned EI.cfg 's then no serial is required and it just install the one preselected, or give you the chance to select the version you want, of course after install you have to apply your serial to validate.

If you are making a clean install in an OEM equipment change Retail for OEM, make shure you select same version (it can be the same+update 1 or 1u1) previously installed on the equipment, you are going to need your certificate too, better make a copy before reinstall. This is a problem when you got Single Language version preinstaled, as it is not easy to find (just MSDN).

Pro VL usually comes single (index 1 of the install.wim file), same apply to Enterprice VL, EI.cfg file like this for Pro VL: